1. Field of the Invention
The present invention relates to an organic electroluminescence display and, more particularly, to an organic electroluminescence display in which an organic light emitting device is formed in a recessed structure.
2. Discussion of the Related Art
Generally, an electroluminescence display is a self-emissive display that electrically excites a fluorescent compound to emit light. These displays are drawing attention as the next-generation display that can overcome drawbacks associated with liquid crystal displays (LCD) regarding viewing angles and response times, among others. They may also be driven at a lower voltage, and they can be made thinner than LCDs.
Electroluminescence displays are classified as inorganic or organic depending upon whether the material comprising the emission layer is an inorganic or organic compound.
The organic electroluminescence display may have an organic layer formed on a transparent substrate such as glass, and a plurality of electrode layers formed on upper and lower portions of the organic layer.
For the organic electroluminescence display described above, when anode and cathode voltages are applied to the electrodes, holes (acting as mobile positive charge carriers) are injected from the anode electrode, and they migrate to an emission layer (EML) via a hole transporting layer (HTL), while electrons are injected from the cathode electrode, and they migrate to the emission layer via an electron transporting layer (ETL). The electrons and holes combine at the EML to generate exitons. As the exitons change from an excited state into a base state, fluorescent molecules of the emission layer emit light, thereby forming an image.
Active matrix (AM) organic electroluminescence displays include at least two thin film transistors (TFTs) per pixel. One TFT is used for driving the pixel while the other controls the pixel's operation. TFTs are typically comprised of drain and source regions with a high concentration of dopant, a semiconductor active layer having a channel region defined between the drain and source regions, and a gate electrode formed on a gate insulating layer. The gate insulating layer is typically formed on the semiconductor active layer, and the gate electrode is typically formed on the channel region of the active layer.
A conventional organic electroluminescence display will now be described with reference to FIG. 1, which shows a cross-sectional view of a conventional organic electroluminescence display using an inorganic passivation layer.
Referring to FIG. 1, a TFT is formed on a lower insulating substrate 110. The TFT is comprised of an active layer 111, a gate electrode 112 and source/drain electrodes 113S and 113D, and it is electrically coupled to a first electrode 114, which serves as anode.
The first electrode 114 is typically formed by depositing a metal such as Ag or Al having good reflectivity, and depositing and patterning a transparent conductive material such as ITO (indium tin oxide) or IZO (indium zinc oxide) having a high work function, thereby forming a transparent anode electrode with a reflective layer.
A pixel defining layer 115 is then formed on the entire surface of the lower insulating substrate 110 and patterned to form an opening 115a, thereby partially exposing a portion of the first electrode.
An organic emission layer 116 is next formed in the opening 115a. 
After forming the organic emission layer 116, a second electrode 117, serving as a cathode electrode, is formed on the entire surface of the lower insulating substrate 110. As a result, an organic light emitting device (OLED) is formed which includes the first electrode 114, the organic emission layer 116, and the second electrode 117.
The second electrode 117 may be formed by depositing a metal such as Al, Ca or Mg having a low work function, and depositing a transparent conductive material such as ITO or IZO.
A second passivation layer 118 is then formed on the entire surface of the substrate.
Finally, an upper insulating substrate 120 is used to encapsulate the lower insulating substrate with the TFT and the OLED.
The space 119 between the upper insulating substrate 120 and the second passivation layer 118 is filled with a gas such as nitrogen (N2) or argon (Ar), or it may be a vacuum. The organic layer is formed by depositing a transparent organic material having the index of refraction of 1.5.
A top-emitting organic electroluminescence display fabricated as described above, however, may lose at least 25% of light because when the light generated at the organic emission layer radiates upward, it is reflected at the interface between the second passivation layer 118 and the second electrode 117, or between the second passivation layer 118 and the space 119 filled with nitrogen. So only 20% of the generated light may be transmitted as a visible image.